Properly sized window overhangs help prevent summer overheating. A mix of good passive design and high-quality active systems make a net-zero success story.

Beginner

Using closed-cell spray foam (usually intended mainly for its insulating properties) on the inside face of all wall and roof sheathing also played a big part in the home’s airtightness. Every wall bottom plate was sealed with a double bead of caulk and a thick rubberized sealer was used between the sill plates and concrete. Every door and window rough opening, and every electrical and plumbing penetration, was sealed with spray foam or caulk. After a 3-inch-thick layer of spray foam was applied to the walls, and before installing the 9 inches of blown fiberglass and the drywall, I conducted a blower door test and used a thermal camera to locate and repair any air leaks. Finally, other than the energy recovery ventilator, there are no fans that vent to the outside—no clothes dryer ducting, or range hood or bathroom vents.

Space Heating

The graph on the following page tracks a particularly cold week in December when the highs and lows for the week were below long-term averages. It shows data for outside, inside, earth tube air, and slab temperatures, in two-hour increments.

Mechanical Heating. After compensating for the passive solar contribution, Energy-10’s predicted annual requirement for mechanical heating was 5,954 kBtu (thousand British thermal units). But we never turned on the minisplit heat pump, and burned only 1.9 cubic feet (0.015 cords) of hardwood molding scraps in a 63% efficient wood heater, which provided 187 kBtu. However, these five one-hour fires were lit for ambiance, not comfort, and we had to open windows to avoid overheating the house.

Earth Tube. Perhaps the most positive surprise is the performance of the 100-foot-long, 8-inch-diameter earth tube, which preheats incoming ERV air in winter and pre-cools the air in summer. This ECOAIR earth tube system has an antimicrobial interior coating, and sealed joints to prevent moisture and radon infiltration.

During design, we had no data on the thermal transfer rate from the ground to the tube air, and the Energy-10 computer model did not include earth tube energy input. So to err on the safe side, I simply ignored the contribution of the earth tube during initial modeling. But the temperature graph shows a fairly significant earth tube contribution. Even with low outside temperatures in the single digits, the incoming tube air maintained a fairly constant 48°F to 49°F. The three temperature spikes on the earth tube air line were due to turning off the ERV and opening some windows to moderate interior temperatures before we installed window coverings. With the ERV turned off, the air in the tube at the temperature sensor begins to rise to interior air temperatures.

Taking one data point in late December, the outside air temperature was 14.8°F, the air temperature in the tube at the house inlet was 48.3°F, and the measured airflow through the 8-inch-diameter tube was 64 cubic feet per minute (cfm). Using the heat delivery rate equation, we estimate the earth tube was producing 2.3 kBtu per hour:

Btu/hr. = sT (°F) × 1.08 (Btu/(hr. × cfm × °F)) × airflow (cfm)

Btu/hr. = 33.5 × 1.08 × 64 = 2,316 Btu/hr. (2.3 kBtu/hr.)

The Energy-10 model predicted a peak heating load of 4.5 kBtu per hour with passive solar gains included. This indicates that, for temperatures in the teens, the earth tube provided approximately 50% of the predicted peak heating load (although the peak load occurs at the design temperature of 5°F).

Another indication of the passive solar contribution to the heating demand was that, with no mechanical backup heating and temperatures as low as -4°F, the coldest interior temperature through the winter was 63°F on two mornings, just before sunrise. By 10 a.m. on those mornings, the temperature was above 68°F.

Solar Hot Water Heat Coil Loop. The original plan was to tap excess heat capacity from the solar hot water storage tank to heat ERV supply air through a water-to-air heat exchanger in the ERV supply duct. We purchased all of the components and installed the heat exchanger, but did not connect it when it became clear that the house was meeting all of its heating load with just passive gain.